For the reduction of pollutants, in particular for the reduction of nitrogen oxides and for the regeneration of diesel particle filters, different methods have become established in which reducing fluids (gases or liquids) are introduced into the exhaust system of an internal combustion engine.
To reduce nitrogen oxides, SCR (selective catalytic reduction) technology has proven to be particularly expedient, in which the nitrogen oxides (NOx) contained in the exhaust gas are selectively reduced, by means of ammonia or a corresponding precursor substance that can be converted to ammonia, to form nitrogen and water. Here, use is preferably made of aqueous urea solutions. The urea solution is hydrolyzed by means of hydrolysis catalytic converters, or directly on the SCR catalytic converter, to form ammonia and carbon dioxide. For this purpose, the urea solution is injected into the exhaust-gas flow upstream of the hydrolysis catalytic converter or the SCR catalytic converter by means of special dosing systems. This results firstly in the problem of determining the optimum reducing agent quantity, but secondly also that of ensuring the reliable supply and dosing of the reducing agent.
If reliable dosing is not ensured, it is not possible to attain efficient removal of nitrogen oxides (NOx) from the exhaust gas. On the one hand, an overdosing of reducing agent can lead to undesired emissions, for example of ammonia, a so-called reducing agent breakthrough. On the other hand, an underdosing can lead to inadequate reduction of the nitrogen oxides. The same also applies correspondingly to the regeneration of diesel particle filters, which takes place by means of an injection of fuel.
In the case of liquid reducing agents, such as the conventional urea solutions or fuel, the dosing may be carried out by means of an injector. Here, the activation time and therefore the opening time of the injector are decisive of the quantity of reducing agent supplied to the exhaust-gas aftertreatment system.
To attain the most effective atomization possible, high injection pressures are used. This simultaneously has the advantage that the ammonia can be released from the reducing agent solution more easily. For this purpose, the injector is connected via a line to the reducing agent pump.
To determine the actual amount of reducing agent injected, the nominal quantity of an injection is set to a constant value, and only the interval times between the injections are varied if required. The constant nominal quantity is set by virtue of a constant activation period of the injector being determined which corresponds to a constant injection quantity. Here, it is important that the same reducing agent quantity can always be expected for a certain activation period. Over the course of time, therefore, as a result of the variation of the interval times between two injections, the injected quantity is always a multiple of the constant injection quantity. This information is important in that there has hitherto been no direct feedback from the system reflecting the actual injection quantity. It is also possible to use a variable activation period of the injector to vary the reducing agent supply. For this purpose, however, the characteristic curve of the injector must be known. It has therefore hitherto not been possible for deviations as a result of production tolerances or system aging to be taken into consideration.
In any case, it is constantly sought to be able to control the injection quantity of reducing agent dispensed through the injection system as effectively as possible during operation so as to permit an injection according to requirements in all operating states.